A variety of methods have been employed for optically pumping solid-state lasers, such as those containing neodymium-doped lithium yttrium fluoride (Nd:YLF) or neodymium-doped yttrium aluminum garnet (Nd:YAG) lasants. A common method is to use an arc lamp or other similar light source to excite a laser rod. The light source and laser rod are positioned within and at different foci of a highly reflective housing of elliptical cross-section. This method typically requires relatively large diameter laser rods to efficiently absorb enough of the pumping light emitted by the light source to allow solid-state laser operation. Another limitation of this pumping method is the relative inefficiency caused by poor overlap of the optical emission spectrum of the pumping light source with the absorption bandwidth of the solid-state lasants. For some industrial operations such as processing electronic materials, compact diode-pumped solid-state lasers offer numerous advantages. For example, large gas lasers or arc-pumped Q-switched YAG lasers that typically require water cooling systems are largely incompatible 17 with clean room conditions often necessary for link processing of dynamic random access memory devices.
There are several different methods for diode-pumping solid-state lasers. In U.S. Pat. No. 3,982,201, Rosenkrantz et al. describe a solid-state laser that is pumped by single diode lasers or arrays of diode lasers to which the solid-state laser rod is end-coupled. Because the output wavelength of the diode laser array is a function of its temperature, the diode lasers are operated in a pulsed mode at a low duty cycle to maintain the array at a sufficiently stable temperature so that its output wavelength remains matched to the absorption bandwidth of the solid-state laser rod. The output power characteristics of this laser system are limited by the relatively inefficient match between the output of the diode lasers and the mode volume of the solid-state laser rod.
In "Efficient LiNdP.sub.4 0.sub.12 Lasers Pumped with a Laser Diode," Applied Optics, vol. 18, No. 23 (Dec. 1, 1979), Kubodera and Otsuka describe the well-known practice of collecting the output light of a diode laser and focusing its expanded output light using conventional lenses, such as two microscope condenser lenses. This method is particularly well suited for applications where the emitter width and beam divergence of the diode laser are small. However, as the emitter dimensions and beam divergence increase, it becomes increasingly difficult to efficiently collect the output beam with collimating lens or lenses. It also becomes more difficult to focus the expanded beam into the solid-state lasant crystal with sufficient depth of focus to allow efficient overlap of the pump beam throughout the resonator mode volume within the lasant.
In U.S. Pat. No. 4,710,940, Sipes, Jr. describes a Nd:YAG solid-state laser that is end-pumped by a diode laser array or by two diode laser arrays that have been combined by use of polarizing beam-splitting cubes. Sipes, Jr., cites the analysis of D. G. Hall in "Optimum Mode Size Criteria for Low Gain Lasers," Applied Optics, 579-1583, vol. 20, (May 1, 1981), to suggest that the "pump profile shape does not matter much as long as all the pump light falls within the resonator mode." Sipes, Jr., notes, however, that Hall's analysis does not account for the divergence properties of Gaussian beams, so Sipes, Jr., suggests that, if required, the cross-section of the pump beam could be modified by use of a cylindrical lens.
In U.S. Pat. No. 4,761,786, Baer describes a Q-switched, solid-state laser that is end-pumped by a diode laser or diode laser array. The output light from the pump source is collected by a collimating lens and directed by a focusing lens to end-pump the laser rod. Baer notes that "other lenses to correct astigmatism may be placed between the collimating lens and focusing lens." Baer also describes an alternate embodiment that employs a remotely positioned diode laser pumping source coupled through an optical fiber, the output of which is focused by a lens into the laser rod.
In U.S. Pat. No. 4,763,975, Scifres et al. describe two optical systems that produce bright light output for a variety of applications, including pumping a solid-state laser such as a Nd:YAG. Scifres et al. describe an optical system that employs a plurality of diode lasers, each of which is coupled into one of a plurality of fiber-optic waveguides. The waveguides are arranged to form a bundle and the light from the diode laser sources is emitted at the output end of the bundle. Optics, such as a lens, may be used to focus the light into a solid-state laser medium. Alternatively, the fiber bundle may be "butt"-coupled to the laser rod. Butt-coupled means end-coupled at a position very close to or in contact with the laser rod.
Scifres et al. describe another optical system that employs a diode laser bar, broad-area laser, or other elongated source to pump a solid-state laser. The diode laser bar light output is coupled into a fiber-optic waveguide having an input end that has been squashed to be elongated and thereby have core dimensions and lateral and transverse numerical apertures that correspond respectively to those of emission dimension and lateral and transverse divergence angles of the diode laser bar. The output light from the fiber-optic waveguide is either focused by a lens into the end of the solid-state laser rod or butt-coupled to the rod. Scifres et al. state that either end of the fiber-optic waveguide can be curved. Although these methods attempt to match the output light from the fiber-optic waveguide to the resonant cavity mode of the solid-state laser, they are limited in efficiency by the numerical aperture of the sources that can be effectively collected and guided by the fiber-optic waveguides.
Certain methods are known for coupling the output of high-power diode lasers into solid-state lasants. High-power diode lasers are necessarily broad-area devices or arrays of narrow-width diode lasers because the potential for catastrophic optical damage to the mirrors dictates that the optical outputs be limited typically to 10 to 20 mW per micron of emission stripe width. Typical high-power diode lasers used to pump solid-state lasants include aluminum gallium arsenide (AlGaAs) diode lasers. Examples of such diode lasers include Model No. SDL-2480-P1 with continuous wave (CW) output power of 3.0 watts (W) and an emission width of 500 .mu.m; Model No. SDL-2462-P1 with CW output power of 1.0 W and an emission width of 200 .mu.m; and Model No. SDL-2432-P1 with CW output power of 0.5 W and an emission width of 100 .mu.m, all of which are manufactured by Spectra Diode Labs, 80 Rose Orchard Way, San Jose, Calif. Use of AlGaAs semiconductor diode lasers to optically pump solid-state lasers has led to development of compact, solid-state lasers.
Broad-area lasers are described by Thompson in "A Theory for Filamentation in Semiconductor Lasers", Optoelectronics, 257-310, vol. 4, (1972) and by Kirkby, et al. in "Observations of Self-Focusing in Stripe Geometry Semiconductor Lasers and Development of a Comprehensive Model of Their Operation," IEEE Journal of Quantum Electronics, 705-719, vol. QE-13 (1977). Such broad-area lasers (emission width of typically greater than 5 .mu.m) exhibit a filamentary structure in their optical nearfield patterns. The filament structures arise from a nonlinear interaction between the carriers and the optical field in the active area of the laser. The process of stimulated emission effectively reduces the gain profile within the active area and results in an increase in the refractive index in the portion of the active area contributing most strongly to the optical mode. This region of increased refractive index is bounded by regions of the active area that do not contribute so strongly to the optical mode and are characterized by smaller refractive index values. This lateral variation in refractive index in a local region within the active area of the diode laser can form a local lateral index guide.
When the active area is broader than about 5-10 .mu.m, as is the case in typical high-power laser diodes used for solid-state laser pumping, several, or in some cases, many such index-guided regions may form. Stimulated emission within each such lateral index-guided region within the active area may occur in the form of a filament that is only partly spatially coherent or is spatially incoherent with respect to neighboring filaments. This filamentation phenomena is, therefore, a fundamental source of lateral spatial incoherence in high-power laser diodes and, consequently, places limits on the optical brightness obtainable from such devices.
Although these methods have with varying degrees of efficiency been used to optically pump solid-state laser mode volumes and been used to produce useful solid-state laser output at a variety of emission wavelengths, improved methods for coupling the optical output from diode lasers into solid-state lasants are highly desirable.
A method for theoretically obtaining high-power, nearly diffraction-limited optical output from a high-power diode laser has recently been described by Tilton, . . . DeFreez, et al., in "High Power, Nearly Diffraction-Limited Output from a Semiconductor Laser with an Unstable Resonator," IEEE Journal of Quantum Electronics, 2098-2108, vol. 27, No. 9, (September 1991). The high-power AlGaAs diode laser described therein demonstrates high power (greater than 1 watt from both facets) and nearly diffraction-limited optical output. The reference states that "[f] or many semiconductor laser applications such as solid-state laser end pumping. . . , single-lobed, diffraction limited beams of hundreds of milliwatts are required." Coupling the optical output from such an unstable resonator into a solid-state laser has not heretofore been attempted, but is described in a concurrently filed U.S. patent application Ser. No. 07/873,411 of Baird and DeFreez for Method and Apparatus for Efficient Operation of a Solid-State Laser Optically Pumped by an Unstable Semiconductor Laser, assigned to assignee of the present invention.
Many important laser applications require laser operation at visible or ultraviolet wavelengths. Moreover, a compact source of coherent visible or ultraviolet light output suitable for use in hospital operating rooms and similar medical environments is also highly desirable for use in a wide range of medical treatments, such as photo-activation of therapeutic drugs.
A variety of methods have been described for generating laser output in the 400 nm to 600 nm wavelength range from solid-state lasers and diode lasers by utilizing the nonlinear process of second-harmonic generation (SHG). For example, several methods have been described for producing SHG laser output in the 520-540 nm wavelength range from diode-pumped, solid-state lasers containing a neodymium-doped lasant. Baer, et al. in U.S. Pat. No. 4,653,056 describe one such method in which an AlGaAs diode laser end-pumps a solid-state laser resonator containing a Nd:YAG rod and potassium titanium phosphate (KTP) nonlinear crystal to produce SHG laser output at 532 nm. As described in "Second Harmonic and Sum-Frequency Generation to 4950 and 4589 A.degree. in KTP," IEEE Journal of Quantum Electronics, vol. QE-24, No. 1 (January 1988), such bulk KTP crystals are phase-matchable for type-2 second-harmonic generation down to 495 nm. For SHG wavelengths shorter than 495 nm, other nonlinear materials are required.
Kozlovsky, et al. in "Efficient Second Harmonic Generation of a Diode-Laser-Pumped CW Nd:YAG Laser Using Monolithic MgO:LiNbO.sub.3 External Resonant Cavities," IEEE Journal of Quantum Electronics, vol. 24, No. 6 (June 1988), describe producing about 30 mW of SHG output at 532 nm by using a diode-pumped Nd:YAG, single-mode ring laser operating at 1064 nm to pump an external monolithic cavity of nonlinear magnesium oxide:lithium niobate (MgO:LiNbO.sub.3).
Another method of producing SHG laser output at 532 nm is described by Schutz, et al. in "Miniature Self-Frequency-Doubling CW Nd:YAB Laser Pumped by a Diode-Laser," in Optics Communications, vol. 77, No. 2, 3 (15 Jun. 1990). Schutz, et al. describe producing a SHG output of about 10 mW at 532 nm by end-pumping a laser resonator containing the self-frequency-doubling lasant neodymium:yttrium aluminum boron (Nd:YAB) with 870 mW emitted by a AlGaAs diode laser array operating at an output wavelength of 807 nm.
Risk and Lenth in "Room-Temperature, Continuous-Wave, 946-nm Nd:YAG Laser Pumped by Laser-Diode Arrays and Intracavity Frequency Doubling to 473 nm," Optics Letters, Vol. 12, No. 12 (December 1987), describe a method to pump a 1 mm length rod of Nd:YAG with two 0.25 W diode laser arrays whose output are combined using a polarizing beamsplitter cube arrangement. The method employs a 5 mm long crystal of lithium iodate (LiIO.sub.3) cut for Type I phased-matched frequency doubling of 946 nm output at room temperature in the solid-state laser resonator cavity and produces approximately 100 .mu.W of SHG blue light at 473 nm. Risk, Pon, and Lenth in "Diode Laser Pumped Blue-Light Source at 743 nm Using Intracavity Frequency Doubling of a 946 nm Nd:YAG Laser," Applied Physics Letters, vol. 54, No. 17 (24 Apr. 1989), describe further work on a similar method employing a single 0.5 W laser diode to end-pump a solid-state laser resonator containing a 1 mm long Nd:YAG rod and a 3.7 mm long KNbO.sub.3 nonlinear crystal to produce. 3.1 mW of blue output at 473 nm.
Methods have also been described in which the laser output from diode lasers are directly frequency doubled. Kozlovsky, et al. describes such a method in "Generation of 41 mW of Blue Radiation by Frequency Doubling of a GaAlAs Diode Laser," Applied Physics Letters, vol. 56, No. 23, (4 Jun. 1990). They employ a monolithic ring resonator of KNbO.sub.3 to convert 105 mW of incident diode laser power at 856 nm to 41 mW of blue 428 nm output power. This method, however, requires use of a diode laser operating in a single longitudinal mode. High-power, gain-guided diode lasers typically do not operate in a single-longitudinal mode and single spatial mode and therefore are not likely to be useful for this method in efforts to achieve higher SHG output powers. In addition, the room-temperature wavelength limit for noncritical phase-matching in KNbO.sub.3 of about 860 nm is likely to prevent SHG wavelengths significantly shorter than the wavelengths they describe from being produced using KNbO.sub.3 and similar methods.
In "Blue Second Harmonic Generation in KTP, LiNbO.sub.3 and LiTaO.sub.3 Waveguides," Phillips Journal of Research, vol. 46, 231-265 (1992), Jongerius, et al. describe conversion of an infrared pump beam into a blue beam by SHG through coupling the pump beam into channel waveguides that have been diffused into the surface of KTP, LiNbO.sub.3, or lithium tantalate (LiTaO.sub.3) substrates. They describe achieving 6 mW of 460 nm output power from a periodically segmented domain-inverted KTP waveguide by pumping the waveguide with 920 nm output from a Ti:Sapphire solid-state laser.
As those skilled in the art will appreciate, AlGaAs diode lasers typically operate within the wavelength range 770 nm to 840 nm. The efficiency of high-power AlGaAs lasers is typically poorer at shorter wavelengths in comparison to higher wavelengths due to active region heating effects. Therefore, conversion of the output power from an AlGaAs diode laser utilizing SHG is likely to be limited to wavelengths greater than 385 nm, unless methods to produce SHG conversion at wavelengths lower than those conventionally demonstrated can be found.
Accordingly, it would be desirable to find a method to produce a compact, diode-pumped solid-state laser which has useful laser output in a more extensive range, such as the 720-920 nm, that can be converted using the nonlinear process of SHG to visible or ultraviolet laser output in the 360-460 nm range.
New chromium-doped, solid-state laser materials such as chromium:lithium calcium aluminum fluoride (Cr:LiCAlF) and chromium:lithium strontium aluminum fluoride (Cr:LiSAlF) have been shown to provide optical output in the 720-920 nm range. These solid-state laser materials are described by S. A. Payne, et al., in "LiCaAlF.sub.6 :Cr.sup.3+ : A Promising New Solid-State Laser Material," IEEE Journal of Quantum Electronics, 2243-2252, vol. 24, No. 11, (November 1988); S. A. Payne, et al., in "Laser Performance of LiSrAlF.sub.6 :Cr.sup.3+," in Journal of Applied Physics, 1051-1055, vol. 66, No. 3; and by S. A. Payne et al. in U.S. Pat. No. 4,811,349.
Such inhomogenously broadened materials have been optically pumped by aluminum gallium indium phosphide (AlGaInP) diode lasers as described by Scheps, et al., in "Cr:LiCaAlF.sub.6 Laser Pumped by Visible Laser Diodes," IEEE Journal of Quantum Electronics, 1968-1970, vol. 27, No. 8 (August 1991) and by Scheps, et al., in "Diode-Pumped Cr:LiSrAlF.sub.6 Laser," Optics Letters, 820-822, vol. 16, No. 11 (Jun. 1, 1991). However, the relatively low stimulated emission cross-section-fluorescence lifetime product of these materials consequently requires relatively large pump powers to obtain laser operation threshold by pumping with such a broad area, high-power diode laser. This requirement results from the relatively large pumping beam radius inherent from the lateral spatial incoherence typical of such devices. The optical output of such a broad-area, high-power diode laser coupled via conventional methods into such an inhomogenously broadened solid-state lasant material is insufficient to generate optical output of usable power from such a solid-state lasant.
Thus, improved methods for coupling the optical output of high-power diode lasers, especially those having improved lateral spatial coherence, into the mode volumes of a solid-state lasant such as Cr:LiCAlF or Cr:LiSAF are highly desirable.